In order to enhance the functionality of a disposable cutter, it is not uncommon to integrate the functions of boring cutters, drill bits, and end mills into a single structure in which a shank can be selectively assembled with a variety of replaceable cutter heads to form different cutting tools. FIG. 6 and FIG. 7 show a conventional multifunctional cutter consisting of a shank 40 and a cutter head 50. The shank 40 is provided with a receiving portion 41 at one end, wherein the receiving portion 41 is a structure with an annular hole section 411 and an internal thread 412. The cutter head 50 may be formed with a disposable milling cutter, a disposable drill bit, or a disposable boring cutter at one end, while the other end of the cutter head 50 is formed as a connecting portion 51, which is a structure with a positioning rod 511 and an external thread 512. The cutter head 50 and the shank 40 can be threadedly connected via the connecting portion 51 and the receiving portion 41 to form a cutting tool whose cutter head 50 can be replaced with those of different functions.
More specifically, the connection between the shank 40 and the cutter head 50 of the foregoing cutter entails the positioning rod 511 being positioned and supported in the annular hole section 411 and the external thread 512 being interlocked with the internal thread 412. However, as the class of fit between a common internal thread and a common external thread is typically a loose fit, the threadedly locked portions of the cutter will wobble slightly when the cutter is in a cutting operation, as explained below. Referring to FIG. 7, when milling horizontally, the milling cutter formed at one end of the cutter head 50 is subjected to a feed load F, which generates a torque M tending to rotate the cutter head 50 about a rotation center C defined by an end portion of the shank 40 (i.e., where the shank 40 is pressed against the shoulder 52 of the cutter head 50). As a result, the cutter head 50 is rotated slightly about the rotation center C in the direction of the torque M and the connecting portion 51 of the cutter head 50 consequently shifts to one side. Now that the cutter works in a rotating manner, the blade which in one moment is in the cutting state (i.e., under load) will be relaxed as soon as it enters the non-cutting state, with the feed load F acting on another blade instead. Thus, the cutter head 50 wobbles as the blades of the milling cutter take turns receiving the feed load F, and the faster the cutter rotates, the higher the wobble frequency. The wobble will, without doubt, result in a shaky cutting operation and leave shaky cutting marks on the cut surface. Moreover, the load on the main shaft of the machine tool will be increased, and cutting depth and cutting speed will be reduced, which compromises processing efficiency and yield.